Train wheels are slightly cone-shaped
You would expect a train wheel to be a straight cylinder, with flat treads.
Category: Getting about
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Feature. The cone steers the train round curves.
Tapered treads let a wheelset steer itself. On a bend the outer wheel rolls on a larger diameter than the inner, so both cover the right distance without slipping. The same taper pulls it back to the centre on straight track.
Source: Railway wheel, Wikipedia.